1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for providing services in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system.
2. Description of the Related Art
Research on the 4th generation (4G) communication system, which is the next generation communication system, is being conducted to provide users with various qualities-of-service (QoS) at a data rate of about 100 Mbps. Currently, therefore, active research on the 4G communication system is being conducted to develop a new communication system that guarantees mobility and QoS for a wireless Local Area Network (LAN) system and a wireless (Metropolitan Area Network) MAN system, thereby to support a high data rate, guaranteed by both systems.
A system employing an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support a broadband transmission network for physical channels of the wireless MAN communication system is defined as an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system. The IEEE 802.16a communication system, because it applies the OFDM/OFDMA scheme to the wireless MAN system, can support high-speed data transmission by transmitting physical channel signals using a plurality of subcarriers. That is, the IEEE 802.16a communication system is one of the typical communication systems using the OFDM/OFDMA scheme (hereinafter referred to as “OFDM/OFDMA communication systems”).
The current IEEE 802.16a communication system takes into account only the single-cell configuration in which mobility of subscriber stations (SSs) is never taken into consideration. However, an IEEE 802.16e communication system is specified as a system that takes into account the mobility of SSs in the IEEE 802.16a communication system. Therefore, the IEEE 802.16e communication system must take into account the mobility of SSs in a multicell environment.
In order to support the mobility of SSs in the multicell environment, there is a need to modify operations of the SS and a base station (BS). In particular, active research is being performed on handover of the SSs for a multicell configuration to support the mobility of SSs. Herein, an SS with the mobility will be referred to as a mobile station (MS).
FIG. 1 is a diagram schematically illustrating a configuration of a conventional BWA communication system. The BWA communication system has a multicell configuration, i.e., has a cell 100 and a cell 150. The BWA communication system includes a BS 110 for managing the cell 100, a BS 140 for managing the cell 150, and a plurality of MSs 111, 113, 130, 151 and 153. Signal exchange between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153 is achieved using the OFDM/OFDMA scheme.
The OFDMA scheme proposed in the BWA communication system creates a subchannel from subcarriers constituting one OFDM symbol, and several OFDM symbols constitute one frame. A format of a frame divided into a plurality of zones in the OFDMA system will now be described with reference to FIG. 2. FIG. 2 is a diagram schematically illustrating a format of an uplink/downlink frame divided into a plurality of zones in a conventional OFDMA communication system.
Referring to FIG. 2, a frame format of the OFDMA communication system includes a downlink (DL) subframe and an uplink (UL) subframe. The downlink frame can include a plurality of zones, such as Preamble, Partial Usage of Subchannels (PUSC), Full Usage of the Subchannels (FUSC), and Adaptive Modulation and Coding (AMC) zones. The PUSC zone, a first-allocated zone of the downlink frame, includes frame allocation information such as a Frame Control Header (FCH) and a DL-MAP, and a change in the subsequent zones can be made using a Space Time Coding (STC) zone Information Element (IE) (STC_ZONE_IE) message of a DL-MAP zone including information on the downlink frame.
The DL-MAP zone, a zone where a DL-MAP message is transmitted, has IEs included in the DL-MAP message. For example, the DL-MAP message includes the STC_ZONE_IE message.
That is, as illustrated in FIG. 2, several resource allocation schemes can exist in one frame, and a change in the zones following the first PUSC zone is made using the STC_ZONE_IE message in the DL-MAP.
A format of the STC_ZONE_IE message is shown in Table 1 below.
TABLE 1SyntaxSizeNotesSTC_ZONE_IE0 {Extended DIUC4 bitsSTC/ZONE_SWITCH=0x01Length4 bitsLength=0x04OFDMA symbol offset8 bitsDenotes the start of the zone (countingfrom the frame preamble and startingfrom 0)Permutation2 bits0b00=PUSC permutation0b01=FUSC permutation0b10=Optional FUSC permutation0b11=Adjacent subcarrier permutationUse All SC Indicator1 bits0=Do not use all subchannels1=Use all subchannelsSTC2 bits0b00=No STC0b01=STC using 2 antennas0b10=STC using 4 antennas0b11=FHDC using 2 antennasMatrix Indicator2 bitsAntenna STC/FHDC matrix (see 8.4.8)0b00=Matrix A0b01=Matrix B0b10=Matrix C (applicable to 4antennas only)0b11=ReservedDL_PermBase5 bitsAMC type2 bitsIndicates the AMC type in casepermutation type = 0b11, otherwiseshall be set to 0AMC type (NxM=N bins byM symbols):0b00 1x60b01 2x30b10 3x20b11 reservedreserved8 bitsShall be set to zero}
Table 1 shows a format of a conventional STC_ZONE_IE message. As shown in Table 1, a Permutation field defines a zone to be allocated after the STC_ZONE_IE. A DL_PermBase field is used for subchannel allocation for each zone. A Use All SC indicator field indicates whether all subchannels are used in a PUSC zone, and this field is disregarded in other allocated zones.
In the STC_ZONE_IE message defining a transmit diversity mode, an STC field indicates a diversity mode based on the number of antennas, and a Matrix Indicator field indicates a type of a matrix encoded using the transmit diversity. Finally, an AMC type field indicates a resource allocation type for an allocated AMC zone (Permutation=0b11).
The PUSC subchannel generation process described above is defined in the 802.16 standard, thus a detailed description thereof will not be given herein for simplicity purpose. Generally, the process defined in the standard generates N subchannels per 2 OFDMA symbols. The generated subchannels have their own unique physical numbers of 0 through N−1.
The 802.16 standard applies a so-called segmentation technique to the PUSC. That is, the standard assigns a unique segment number to each cell or sector, and the cell or sector allocated the segment number exclusively uses only a part of the generated full PUSC subchannel. In other words, a start point of an allocated subchannel is determined according to the segment number.
The number of the allocated subchannels is determined depending on the FCH. In the current standard, a segment number indicating a start point of a subchannel is determined depending on a segment number of a preamble. That is, if a preamble is detected for acquisition of synchronization between MSs, a segment number to be used in a frame where the preamble is transmitted is automatically acquired. The current standard defines three values of 0, 1 and 2 as preamble segment numbers. In the following description, the segment number will be referred to as a “segment indicator.”
In summary, allocation of PUSC subchannels is achieved by generating the PUSC subchannels, applying the segmentation technique to the generated PUSC subchannels, and subsequently, applying a renumbering technique to the segmented PUSC subchannels. An exemplary method of applying the renumbering technique to the PUSC subchannels will now be described with reference to FIG. 3.
FIG. 3 is a diagram illustrating a renumbering method for a PUSC zone in an OFDMA communication system according to the prior art. Numbers of logical subchannels (logical enumeration (renumbering)) 305 obtained by re-defining numbers of physical subchannels (physical enumeration) 303 are acquired from a segment indicator 301 obtained through the preamble. The acquired logical subchannel numbers 305 have subchannel numbers beginning from 0 regardless of the segment indicator.
The resource allocation scheme in the OFDMA communication system has been described so far. Next, a single-frequency network (SFN) service will be briefly described. In a general multi-frequency network (MFN), repeaters are installed in every service area to provide services transmit signals by channel switching. The MFN requires many frequencies to provide a broadcast service. Research has been conducted on technology for enabling neighboring service areas to use the same transmission frequency using a characteristic that the OFDM system is robust against multipath channels. This scheme is called a single-frequency network (SFN). The SFN allows neighboring BSs to transmit the same data over the same subchannel thereby to acquire a transmit diversity effect, thus guaranteeing high reception performance.
The SFN service in the OFDMA communication system can be achieved through a zone defined by the STC_ZONE_IE message described with reference to Table 1, after the first PUSC zone. This is because the first PUSC zone includes FCH and DL/UL-MAP indicating the frame allocation information.
If it is assumed that a resource allocation scheme defined by the STC_ZONE_IE message is specified as a PUSC scheme, an MS performs renumbering. However, an MS mapped to its associated segment decodes the subchannels allocated such that they are logically identical to each other but physically different from each other. In this case, therefore, the MS cannot receive the SFN service. This will be described below with reference to the accompanying drawing.
FIG. 4 is a diagram illustrating a renumbering method for an SFN service in an OFDMA communication system according to the prior art. It is assumed in FIG. 4 that 30 subchannels are allocated for the SFN service in a conventional PUSC zone. MSs perform renumbering operations 407, 409 and 411 from segment indicators 401, 403 and 405 for their associated segments. In this case, the logical subchannels are equal in order but different in physical position, making a normal SFN service impossible.